Bio 169 Endocrine System Study Guide
Spring Semester 2005
Learning Objectives, Topics, and Key Terms
Readings: Martini, Ch. 18, pp 605 - 634.
Overall view of hormones:
exocrine vs endocrine secretion (KEY CONCEPT)
endocrine glands are ductless (their cells secrete across the basal membrane -- what is exocrine?)
hormones are chemical signals secreted by endocrine glands
hypophysis (also called, pituitary; intimately related to hypothalamus )
thyroid
parathyroid
pancreas
adrenal
gonads
also from endocrine cells found within most other organs
adipose tissue
brain
gut
kidney
heart
placenta
Comparison of Nervous and Endocrine Systems
nerve cells release chemical (neurotransmitter) locally at synapse
rapid effect (e.g., movement; sensation)
endocrine cells release chemical (hormone) into blood stream
action occurs at some distance (ususally)
slower effect (e.g., lowering of blood sugar)
coordinated action of both systems (i.e., the "neuroendocrine" system)
Hormone Action
affect target cells only - HOW?
by binding to specific receptors with high affinity
receptors decrease if a lot of hormone ("down regulation")
receptors increase if a not much hormone ("up regulation")
hormones are inactivated by one of several mechanisms
a circulating enzyme, or chemical modification in liver, or excretion by kidney
Endocrine vs Paracrine vs Autocrine
distant target cell (by circulation) vs local target cell (by diffusion) vs on self
Hormone Chemistry
steroids
derived from cholesterol
e.g., estrogen, cortisol
biogenic amines (amino acid derivatives)
e.g, epinephrine, histamine, thyroid hormone
peptides and proteins
e.g., ADH (a peptide), insulin (a protein)
eicosanoids
e.g., prostaglandins
water-insoluble hormones travel in blood mostly bound to specific transport proteins
Mechanisms of Hormone Action
target cells must have specific receptors for hormone
properties of the target cell determines the response to a hormone
e.g., insulin --> storage of glycogen in hepatocytes but storage of triglycerides in adipocytes
Intracellular receptors
e.g., nuclear receptors turn on genes in DNA --> mRNA --> particular proteins
lipid soluble hormones like thyroid and steroids can reach intracellular receptors
Extracellular receptors
water soluble hormones such as epinephrine or insulin can't enter cells
hormone = first messenger -- binds to extracellular receptor
hormone binding to receptor is signaled by a G-protein
intracellular chemical = second messenger
example: cyclic AMP (abreviation: cAMP)
made by adenylate cyclase
cAMP activates protein kinases
kinase phosphorylates target protein -- adds a phosphoryl group
turns on or off the protein (e.g., activates an enzyme or opens a channel)
protein phosphatase removes phosphoryl group to end hormone effect
cAMP destroyed by phosphodiesterase
initiates a cascade or chain reaction leading to amplification of hormone effect
Control of Hormone Secretion
by nervous system
e.g., epinephrine (by preganglionic fibers of S-ANS)
by another hormone
e.g, pituitary hormones -- TSH --> thyroid
by blood level of metabolite
e.g., parathyroid hormone (low calcium ion in plasma triggers secretion of PTH)
Hypothalamus and Hypophysis (Pituitary)
hypophysis located in hollow of the sphenoid bone called "sella turcica"
hypothalamus controls hypophysis (main neuroendocrine link)
anterior lobe of pituitary is glandular (adenohypophysis)
controlled by releasing and/or inhibiting factors made in hypothalamus
posterior lobe of pituitary is neural (neurohypophysis)
made of unmyelinated axons with cell bodies in hypothalamus
blood supply to adenohypophysis is via a portal system: the hypothalamohypophyseal portal system
superior hypophyseal arteries form a primary plexus in the hypothalamus
portal veins pass down pituitary stalk
form a secondary plexus in adenohypophysis (anterior lobe)
delivers hypothalamic hormones that control adenohypophyseal cells
hypothalamic neurons secrete hornones that act on adenohypophyseal cells to cause them to
increase or decrease secretion of their hormone (specific to each cell type)
enter primary plexus, travel in portal veins down the stalk, leave secondary plexus
can be releasing hormones (e.g. GHRH) or inhibiting hormones (e.g., GHIH or somatostatin)
five cell types in adenohypophysis
somatotrophs - human growth hormone (hGH; also, somatotropin)
lactotrophs - prolactin - milk production
corticotrophs - adrenocorticotropic hormone (ACTH or corticotropin) - stimulates adrenal cortex to release cortisol, a "glucocorticoid"
thyrotrophs - TSH - stimulates thyroid gland
gonadotrophs - LH and FSH - act on gonads
negative feedback regulation - thyroid hormone as example
plasma level of T3 rises -->
more T3 bound to T3 receptors in the nuclei in TRH and TSH cells in hypothalamus and ant. pit. -->
more inhibition of specific DNA synthesis coding for these -->
less secretion of TRH and TSH -->
plasma level of T3 falls --> less inhibition of specific DNA synthesis in those cells --> et cetera
Growth Hormone (also called somatotropin)
actions: increase amino acid uptake and protein synthesis; release triglycerides from adipocytes (lipolysis); release glucose from liver
promote skeletal growth, growth of other organs
effects may be direct or by release of somatomedins (growth factors) by liver
pathology: too little -> pituitary dwarfism; too much -> pituitary giantism or acromegaly
Posterior Pituitary (Neurohophysis)
hormones are first synthesized in hypothalamic neurons (peptides)
released in neurohypohysis at axon terminals when action potentials pass down axons
antidiuretic hormone (ADH) - promotes water retention in kidney
pathology: diabetes insipidus from insufficiency (e.g., after brain injury)
oxytocin
contracts uterine smooth muscle at parturition
pit drip (IV infusion of Pitocin®) used to induce labor
contracts myoepithelial cells of mammary glands to force milk out
neuroendocrine reflex (suck nipple --> oxytocin secretion --> "letdown")
Thyroid Gland
Thyroid Hormone (TH is a mixture of two separate substances:T3 and T4)
Synthesis
iodide trapping by powerful iodide pump
thyrocytes secrete "colloid" into follicle lumen (colloid =thyroglobulin + peroxidase + iodide
T3 and T4 form on thyroglobulin.
TSH activates pinocytosis and digestion of thyroglobulin in lysosomes.
--> T3 and T4 released in blood.
regulation: negative feedback onto hypothalamus and thyrotrophs
actions: increase heat production by increasing number of Na-K pumps which turn ATP into heat
promote growth and development (especially of nervous system); up regulate adrenergic receptors --> fast heart rate if too much TH
pathology: hypothyroidism - too little (from insufficient dietary iodide, for example) -->goiter
cretinism
hyperthyroidism - too much (from Graves disease for example) --> thyrotoxicosis
can destroy bad thyroid with radioactive iodine (radiothyroidectomy)
Calcitonin - secreted by C cells (parafollicular cells). Lowers plasma Ca++ by activating osteoblasts. C cells respond directly to eleveated calcium ion levels in plasma.
Parathyroid Gland
2 pairs on back of thyroid - very small
secrete parathyroid hormone (a protein) from chief cells
secretion controlled by plasma calcium ion levels
if Ca2+ falls, secretion of PTH goes up
action: increase osteoclastic activity in bone to release calcium, increase calcium retention in kidney, promote calcium uptake from gut
pathology: insufficiency leads to hypocalcemic tetany
Endocrine Pancreas
islets of Langerhans, small balls of endocrine cells amongst exocrine tissue
beta cells secrete insulin
alpha cells secrete glucagon
actions of insulin: lower plasma glucose levels by increasing glucose entry in cells with
insulin receptors. main target cells: liver (glycogen formation), muscle (ditto), adipocytes (triglyceride synthesis)
actions of glucagon: raise plasma glucose, from liver, by promoting glycogenolysis
control of secretion: alpha and beta cells sense plasma glucose directly
pathology: insufficient insulin or insulin receptors leads to diabetes mellitus
high blood glucose, glucose in urine, polyuria, polydipsia, signs of cellular starvation, damage to endothelium by glucose reacting with tissue proteins (glycosylation - measure hemoglobin)
Adrenal Gland (Cortex and Medulla)
anatomy: on top of kidney; blood supply is arterial plexus; connective tissue capsule
cortex has three cell layers: all secrete steroids; synthesized from cholesterol
zona glomerulosa (aldosterone, a mineralocorticoid)
zona fasciculata (cortisol, a glucocorticoid)
zona reticularis (androgens)
actions: aldosterone conserves sodium and promotes potassium loss in kidney
cortisol increases blood glucose from liver by promoting gluconeogenesis
androgens promote blood production, support libido (important in woman)
control of secretion: aldosterone - serum K+ has directly stimulates release; also, by angiotensin II
cortisol - by ACTH (negative feedback of cortisol on corticotrophs and hypothalamic CRF cells)
medulla - postganglionic sympathetic neurons is embryonic source
"cell bodies without axons"
epinephrine (also called adrenaline) is main hormone
actions: increase heart rate and power, dilate bronchial tree, mobilize sugar and fatty acids from liver and adipocytes - cAMP based control system
control of secretion: impulses in preganglionic sympathetic axons trigger release
Endocrine System Study Aid
Complete this table to organize your study of endocrines. Bring to exam!
Hormone |
Cellular Source |
Target Cell(s) |
Action(s) |
Regulation of Secretion |
hGH |
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TSH |
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ACTH |
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Prolactin |
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FSH |
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LH |
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ADH |
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Oxytocin |
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T 3, T4 |
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Calcitonin |
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PTH |
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Aldosterone |
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Cortisol |
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Adr. Androgens |
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Epinephrine |
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Glucagon |
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Insulin |
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Durham Technical Community College
Durham, NC 27703
Last updated 12 December 2005